High power microwave low-pass filter of the leaky wall type

ABSTRACT

A high power microwave low-pass filter of the &#39;&#39;&#39;&#39;leaky wall&#39;&#39;&#39;&#39; type is disclosed. The filter employed in the output circuit of the high power microwave source such as a klystron, magnetron, or the like between the source and the load to suppress the harmonic output from the microwave source. The microwave filter comprises a section of main waveguide which is typically rectangular provided with flanges on its opposite ends for connection to the source and the load, respectively. An array of secondary waveguides, which are dimensioned to be below cutoff for the fundamental frequency of the microwave energy to be passed by the filter are coupled through the walls of the primary waveguide to the microwave energy in the primary guide. The secondary waveguides are provided with resistive card attenuators for absorbing the microwave harmonic energy coupled from the primary waveguide to the attenuators via the secondary waveguides. The secondary waveguides are rectangular hollow waveguides having at ;east nearly equal height and width dimensions such that the seo secondary waveguides will support cross polarized microwave energy in the second harmonic range of the primary waveguide whereby attenuation of the harmonic content of the microwave energy in the primary waveguide is enhanced. The wl walls of the primary guide are defined by the inner ends of the secondary rectangular waveguides. The resistive card attenuators are disposed on a diagonal of the secondary waveguides for coupling to all the possible modes of the secondary waveguides. In one embodiment, the planes of the walls of the secondary waveguides are disposed at substantially 45* to the longitudinal axis of the primary guide to enhance coupling of the secondary waveguides to the fields in the primary waveguide.

United States Patent [72] Inventor John P. Rooney Pale Alto.alit. [2 1] Appl. No. 744,806 r [22] Filed July I5, 1968 [4S] Patented July 13,197! [73] Assignee Varlan Associates Palo Alto, Calif.

[54] HIGH POWER MICROWAVE LOW-PASS FILTER OF THE LEAKY WALL TYPE 10 Claims, 15 Drawing Figs.

[52] 11.8. CI .7 333/73, 333/8l, 333/98 [51] InLCI r. "01p 1/16, HOlp 1/22. H03b 7/l0 [SOI Field ofSeareh a. 333/7173 W,98,98 M,2l,8l

[56] References Cited UNITED STATES PATENTS 3.3$3,|23 ll/l967 Met 333/73 3,451,014 6/l969 Brosnahan et at. 333/73 3,464.035 8/1969 Van Kol 333/8] X Primary Examiner-Herman Karl Saalbach Assistant Examiner-Marvin Nussbaun Attorneys-William J. Nolan and Leon F Herbert ABSTRACT: A high power microwave low-pass filter of the "leaky wall" type is disclosed. The filter employed in the output circuit ot'the high power microwave source such as a klys tron, magnetron, or the like between the source and the load to suppress the harmonic output from the microwave source. The microwave filter comprises a section of main waveguide which is typically rectangular provided with flanges on its opposite ends for connection to the source and the load, respectively. An array of secondary waveguides, which are dimensioned to be below cutoff for the fundamental frequency of the microwave energy to be passed by the filter are coupled through the walls of the primary waveguide to the microwave energy in the primary guide. The secondary waveguides are provided with resistive card attenuators for absorbing the microwave harmonic energy coupled from the primary waveguide to the attenuators via the secondary waveguides. The secondary waveguides are rectangular hollow waveguides having at ;east nearly equal height and width dimensions such that the sec secondary waveguides will support cross polarized microwave energy in the second harmonic range of the primary waveguide whereby attenuation of the harmonic content of the microwave energy in the primary waveguide is enhanced. The wl walls of the primary guide are defined by the inner ends of the secondary rectangular waveguides. The resistive card attenuators are disposed on a diagonal of the secondary waveguides for coupling to all the possible modes of the secondary waveguides. in one embodiment, the planes of the walls of the secondary waveguides are disposed at substantially 45 to the longitudinal axis of the primary guide to enhance coupling of the secondary waveguides to the fields in the primary waveguide.

PATENIED JUL 1 31911 3; 593; 220

SHEEI 1 B? 3 j JOHN P. ROONEY H BY HTTORNEY PATENTED JUL 1 a IBTI SHEET 2 [IF 3 FIG. 7

FIG.8

FIG. IO

1'4 l'a FREQUENCY (cm) INVENTOR. JOHN P. ROONEY BY M 2 TORNEY Ill-l HIGH POWER MICROWAVE LOW-PASS FILTER OF THE LEAKY WALL TYPE DESCRIPTION OF THE PRIOR ART Heretofore, high power microwave low-pass filters of the "leaky wall type have been constructed. One such filter employed an array of rectangular secondary waveguides coupled to the primary guide through elongated coupling irises provided in the wall of the primary guide. Resistive card attenuators were provided at the ends of the rectangular secondary waveguides, such cards being centrally disposed of the secondary guides and extending from one broad wall to the other broad wall thereof. One problem with this prior art microwave filter was that the centered resistive card attenuators provided a relatively poor termination for the TE mode in the secondary waveguide so that energy in this mode could pass through the secondary waveguide to the space surrounding the filter which was generally bounded by a metallic shield from whence it could couple back to the primary waveguide at the output end of the filter with relatively little attenuation. Moreover, the elongated coupling slots provided relatively poor coupling to the TE mode and as a consequence the attenuation of the TIL, mode above the second harmonic was relatively poor. The elongated coupling slots in the broad wall coupled well to the TE, mode but poorly to the TE, mode and hardly at all to the TE mode. Some prior art filters of the aforementioned type included additional longitudinally directed cou ling slots near the corners of the primary guide for coupling to the longitudinal magnetic fields of the modes. However, such longitudinal slots were not feasible when the operating frequency is relatively low in the primary waveguide band because there was no room for the longitudinal slots owing to the large width of the secondary waveguides. In addition, the provision of the longitudinal slots considerably complicated the construction of the filter.

In another prior art filter, which is described in the IRE Transactions on Microwave Theory and Technique, on Nov. I962, pages 428-43l, the secondary waveguides comprised circular waveguides completely filled with dielectric quartz rods to permit the secondary waveguides to be small enough to be packed two or three abreast across the broad wall of the primary waveguide while at the same time being capable of propagating second harmonic signals. While this latter prior art design provided improved attenuation for the higher harmonies it substantially complicated the construction of the filter thereby substantially increasing the manufacturing costs and weight thereof.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an improved high power microwave low-pass filter of the "leaky wall" type.

One feature of the present invention is the provision of an array of secondary waveguides having at least equal height and width and dimensioned such that the secondary waveguides will support cross polarized microwave energy at approximately the same frequency within the second har monic range of the primary waveguide. whereby coupling to and attenuation of the harmonic content of the microwave energy in the primary waveguide is enhanced.

Another feature of the present invention is the same as the preceding feature wherein the secondary waveguides are resistively terminated by means of a plurality of resistive attenuative cards disposed diagonally across and within the secondary waveguides. whereby all of the modes which may be supported in the secondary waveguides are heavily coupled to the attenuators.

Another feature of the present invention is the same as any one or more of the preceding features wherein the secondary waveguides are of square cross section and wherein the planes of the walls of the secondary waveguides are disposed at substantially 45 to the longitudinal axis of the primary guidesuch that three rows of square cross section and two rows of triangular shaped cross section secondary guides are coupled to each of the broad walls of the rectangular primary waveguide, whereby increased coupling to the harmonic content of the wave energy within the primary guide is enhanced.

Another feature of the present invention is the same as any one or more of the preceding features wherein the rectangular secondary waveguides at both ends of the array of secondary waveguides are half the height of the other secondary guides for impedance matching the primary waveguide to the source and to the load, respectively.

Another feature of the present invention is the same as any one or more of the preceding features wherein the height of the primary waveguide is progressively decreased toward the center thereof from both ends for impedance matching the primary waveguide to the source and to the load respectively, while achieving increased harmonic attenuation.

Other features and advantages of the present invention will become apparent on a perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic perspective view, partly broken away, depicting a typical prior art microwave low-pass filter of the "leaky wall" type;

FIG. 2 is a longitudinal top view of a portion of the structure of FIG. I taken along line 2-2 in the direction of the arrows;

FIG. 3 is a longitudinal view of a side portion of the structure of FIG. 1 taken along line 3-3 in the direction of the arrows;

FIG. 4 is a longitudinal view of a prior art comb attenuator;

FIG. 5 is a fragmentary view similar to that of FIG. 2 depicting an alternative prior art secondary waveguide array wherein the secondary waveguides are circular and filled with dielectric;

FIG. 6 is a view similar to that of FIG. 3 depicting an alternative prior art array of secondary waveguides coupled to the narrow wall of the primary guide and comprising circular waveguides filled with dielectric;

FIG. 7 is a view similar to that of FIGS. 2 and 5 depicting the secondary waveguide array of the present invention;

FIG. 8 is a view similar to that of FIGS. 3 and 6 depicting a secondary waveguide array as coupled to the narrow wall of the primary guide and incorporating features of the present invention;

FIG. 9 is a transverse sectional view similar to that taken along line 9-9 In the direction of the arrows of FIG. I and depicting the filter construction of FIGS. 7 and 8 of the present invention;

FIG. I0 is a plot of attenuation in db. versus frequency depicting the stopband attenuation characteristics of a filter incorporating features of the present invention;

FIG. II is a view similar to that of FIG. 7 depicting an alter native embodiment of the present invention;

FIG. 12 is a view similar to that of FIG. 8 depicting an alternative embodiment of the present invention;

FIG. 13 is a view similar to that of FIG. 9 depicting an alternative embodiment of the present invention;

FIG. 14 is a plot of attenuation in db. versus frequency depicting the stopband attenuation characteristics of the filter of FIGS. ll--- 13, and

FIG. 15 is a longitudinally foreshortened sectional view of the structure of FIG. 9 taken along line 15-15 in the direction of the arrows.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. I, there is shown a high power microwave low-pass filter I of the "leaky wall" type. The microwave low-pass filter I includes a primary rectangular waveguide 2 fitted with flanges 3 and 4 at its ends for connection to a source of microwave energy, not shown, which is rich in harmonic content, such as a high power klystron amplifier,

a traveling wave tube or magnetron, and for connection at its other end to a suitable load, such as an antenna, not shown. An array of secondary waveguides 5 is coupled to the primary waveguide 2 via the intermediary of a plurality of elongated coupling slots 6 communicating through the walls of the primary waveguide 2.

The array of secondary waveguides 5 includes two rows of rectangular secondary waveguides coupled to each of the broad walls of the primary waveguide 2 and a row of secondary waveguides coupled to each of the narrow walls of the primary guide 2. The prior art secondary waveguides 5 are of standard rectangular configuration. i.e., having a height approximately one half of their width, such waveguides are arranged with the broad walls of the secondary guides perpendicular to the longitudinal axis of the primary guide 2. The secondary waveguides 5 are dimensioned to support a dominant TE mode at the second harmonic of the fundamental passband of the primary guide 2. A resistive tapered attenuation card 7 is centrally disposed at the end of each of the rectangular secondary guides 5 and projects into the waveguides 5 from the outer open ends thereof for absorbing microwave energy coupled into the secondary waveguides S. A hollow cylindrical metallic housing 8 is coaxially disposed of the primary guide 2 and encloses the primary and secondary waveguides to prevent stray radiation of microwave energy coupled out of the primary waveguide 2 via the secondary waveguides 5.

Referring now to FIGS. 2 and 3, the arrangement of the prior art secondary waveguides 5 is more fully disclosed. FIG. 2 depicts the arrangement of the secondary waveguides 5 and the coupling slots 6 on the broad walls of the primary guide 2. At both ends of the broad wall array of secondary waveguides the coupling holes 6 are progressively elongated taken in a direction from the end toward the center of the array for improved impedance matching of the primary waveguide 2 to the source and load, respectively.

Referring now to FIG. 4, the comb shape resistive card attenuator 7 is more fully disclosed. The resistive card 7 includes a plurality of fingers which are centrally disposed of each of the secondary waveguides 5. The fingers project into the secondary waveguides from their outer open ends. The resistive card 7 is preferably made of an asbestos or glass fiberplastic material impregnated with carbon.

In operation, the TB mode in the primary guide, which corresponds to the second harmonic in the primary guide, couples well to the secondary waveguides 5 on the broad walls of the primary guide 2, whereas the coupling of the secondary waveguides on the broad wall is less effective to TE mode in the primary guide which corresponds to the third harmonic. As to the fourth harmonic, the coupling of the secondary guides 5 to the TE mode in the primary guide is relatively weak since this harmonic corresponds to the TB mode in the secondary guides which provides an electric field null at the position of the attenuating comb structure 5.

The secondary guides S on the narrow wall of the primary guide 2 couple well to the TIEL,, mode in the primary guide. However, the TE mode is attenuated less than the TB mode because the coupling fields are weaker and because there are only half as many secondary waveguides on the narrow walls that can couple to this mode. As a consequence, TE,, attenuation is poor above the second harmonic.

Referring now to FIGS. 5 and 6, there is shown an altemative prior art arrangement of secondary waveguides wherein the secondary waveguides are formed by closely packed cylindrical bores in a copper block to form cylindrical waveguides 9 which are filled with a dielectric material such as quartz. The cylindrical dielectric loaded secondary waveguides 9 are arranged in a pattern of 2 and 3 abreast on the broad walls of the primary guide 2 and in a pattern of two staggered rows on the narrow walls of the primary guide 2 as shown in FIG. 6. A microwave filter configuration of--FIGS. 5 and 6 which employs the dielectrically loaded cylindrical secondary waveguides constitutes the prior art filter design described in the aforecited IRE Transactions on Microwave Theory and Technique. This prior art filter has been found to provide a relatively high attenuation for the second, third and fourth harmonic in the primary guide 1 but results in a relatively complicated and heavy filter construction having a relatively high cost of manufacture. The outer ends of the cylindrical secondary waveguides 9 are terminated in a resistive load material by permitting the dielectric quartz rods to project from the outer ends of the cylindrical bores 9 into and to be embedded in a lossy epoxy material which fills the space between the housing 8 and the secondary waveguides 9.

Referring now to FIGS. 7-9, there is shown the arrangement of primary and secondary waveguides in the low-pass microwave filter l of the present invention. More specifically, the secondary waveguides 5 are rectangular hollow waveguides having at least nearly equal height and width dimensions such that the secondary waveguides! will support dominant mode, cross polarized microwave energy at approximately the same frequency within the second harmonic frequency range of wave energy within the passband of the primary guide 2, whereby coupling to and attenuation of the harmonic content of the microwave energy in the primary waveguide 2 is enhanced. The square or nearly square secondary waveguides are arranged in two linear rows on the top and bottom broad walls of the primary guide 2 and a single row on each of the narrow walls of the primary guide 2. The secondary waveguides 5, at opposite ends of the array, are dimensioned to have half the height of those secondary guides disposed intermediate the ends to provide improved impedance matching between the primary guide 2 and the source and load, respectively.

Each of the secondary guides 5 is terminated via a resistive card, such as carbon impregnated asbestos or Silicon Carbide sheet which is diagonally disposed of each of the square or nearly squsre secondary guides 5. When the resistive card attenuator I5 is disposed diagonally of the square or nearly square rectangular secondary waveguides 5 the attenuator I5 is coupled to any possible mode which is capable of being supported within the secondary guides 5.

As in the prior art filters l, the secondary waveguides 5 are dimensioned to be cutoff for the fundamental wave energy within the passband of the primary guide 2 and are preferably dimensioned to be slightly above cutoff for the second harmonic of the wave energy within the primary guide. By dimensioning the secondary guides 5 to be of square or nearly square cross-sectional dimension these guides are capable of supporting two cross polarized modes in the second harmonic range of the primary guide 2. This means that the TB harmonic modes are attenuated by the broad wall secondary waveguides 5 as well as by those on the narrow sidewalls. The attenuation of the harmonics of the T15 mode are similarly enhanced. The narrow wall secondary waveguides 5 also serve to attenuate the primary TE and the TIE modes which are weakly attenuated by the broad wall secondary waveguides 5 because of their location. The square secondary waveguides also couple well to the longitudinal magnetic fields of the primary harmonic TE modes near cutoff. Metallic covers 20 close off the outer open ends of the secondary waveguides S to permit pressurization of the filter l with gas and to prevent leakage of microwave energy out through the open ends of the secondary guides 5.

Fabrication of the filter l employing the square or nearly square cross-sectional secondary guides 5 is facilitated since the secondary guides are arranged in four modules which are brazed together at their adjoining side edges at 16 such that no slotted primary wa eguide 2 is required. Instead the inner edges of the secondary waveguides 5 form the primary waveguide structure. Because of this modular construction it is relatively easy to taper the height of the primary waveguide 2 in the manner shown in FIG. 15 by tapering the inner edges of the secondary guides which define the top and bottom broad walls of the primary guide 2. By progressively decreasing the height of the secondary guide 2 from both ends thereof toward the center an improved impedance match is obtained to the source and to the load, respectively. In addition, the attenuation of the undesired harmonics is thereby increased.

In some embodiments of the filter l of the present invention, the secondary waveguides 5 are not square. In these embodiments the transverse walls of the secondary guides 5 which are normal to the longitudinal axis to the primary guide 2 are increased in width so as to utilize more of the available spacev However, the short or longitudinal dimension of the secondary guides 5 remains sufficiently large to insure the existence of two crossed polarized modes in the second harmonic range of the primary guide 2. This differs from the prior art filter 1 employing rectangular secondary waveguides in that in the prior art these secondary waveguides were not dimensioned to permit the existence of two cross-polarized modes in the second harmonic range of the primary guide 2, but would support only one mode with the electric field normal to the broad walls to the secondary guides 5.

Referring now to FIG. 10, there is shown the stopband characteristic of the low-pass high-power microwave filter 1 of the type shown in FIGS. 7-9. The characteristic shows relatively high attenuation for the harmonics of the primary T13 mode to well above the third harmonic. It is to be noted that the TE attenuation in the stopband drops ofl' substantially at the third harmonic. This drop off in the attenuation at the third harmonic appears to be due to a decrease in a coupling to the secondary waveguide when the transverse dimensions of the secondary waveguides 5 are equal to a wavelength in the primar waveguide 2. it has been found possible to shift these low points in the attenuation to points higher in frequency by making the secondary waveguides 5 somewhat smaller, so that for narrow band applications the holes in the attenuation characteristic are just above the third harmonic.

Referring now to FIGS. 11-13, there is shown an alternative low-pass microwave filter configuration of the present invention. ln this embodiment, the secondary waveguides 5 are of a square cross-sectional dimension with the planes of the walls of the secondary guides 5 disposed at substantially 45 to the longitudinal axis of the primary guide 2.

This diagonal pattern of the secondary waveguides is less selective in the attenuation of modes in the primary waveguide 2 as compared to the rectilinear pattern of FIGS. 7-9. More particularly, the staggered locations of the apertures of the secondary waveguides 5 enhances coupling to the primary modes with three or four half sine wave variation across the broad wall of the primary guide 2. Thus, enhanced coupling is obtained to the primary TE, and TE modes. In addition, the meandering of the current carrying webs, which define the walls of the primary guide, prevent propagation of any particular mode with low attenuation. That is to say, continuous mode conversion will occur because the boundary condition for the primary rectangular waveguide modes are constantly changing with axial distance within the primary guide and some of the modes or fields so produced coupled to the secondary guides 5.

Resistive card attenuators 15 are provided in each of the secondary waveguides 5 with the card extending diagonally across the square crosssectional secondary waveguides 5. lt will be noted that in the diagonal array of FIGS. 11-13, there results two rows of triangular shaped secondary waveguides 5 at the outer edges of the row of square cross-sectional waveguides 5. These triangular cross-sectional waveguides are terminated by means of resistive cards disposed with the plane of the card perpendicuiar to the base leg of the triangular cross-sectional guide with the card l5 bisecting the apex angle of the triangle.

Referring now to FIG. 14 there is shown a plot of attenuation in db. versus frequency depicting the stopband attenuation characteristic of the filter 1 employing a diagonal array of secondary waveguides 5 as shown in FIGS. 11-13. Except for some raggedness in the lower portion of the third harmonic, TE mode attenuation is high up to the seventh harmonic. TE harmonic mode attenuation of the diagonal filter is also superior at the high harmonics, although the raggedness in the lower part of the third harmonic is undesirable. in the diagonal pattern of secondary waveguides 5, for the same web thickness there is about 40 per cent more metal area for the primary waveguide 2 than that obtained by the rectilinear array of secondary guides shown in FIGS. 7-9. This permits thinner webs to be employed in the diagonal design filter for a filter having the same power handling capacity as a given rectilinear filter 1. Reducing the thickness of the webs allows the diagonal filter to achieve greater stopband attenuation. The primary advantage to be gained by the use of the diagonal array of square secondary waveguides is that the filter is not nearly as selective of primary waveguide modes and provides high attenuation, especially at high harmonic frequencies.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What I claim is:

1. in a microwave low-pass filter of the leaky wall type, means forming a hollow primary waveguide adapted to be connected between a source of high power microwave energy having substantial undesired harmonic content and a load for transmission of the primary waveguide energy within a certain passband of microwave energy to the load, means forming an array of secondary waveguides dimensioned to be below cutofl' for the primary band-pass frequency energy and coupled through the walls of said primary waveguide to the microwave energy in said primary waveguide for extracting harmonic microwave energy of the primary guide therefrom, means forming a lossy termination for said secondary waveguides for absorbing the harmonic microwave energy coupled from said primary waveguide via said secondary waveguides, the improvement wherein, said secondary waveguides are rectangular hollow waveguides having at least nearly equal height and width dimensions such that said secondary waveguides will support crossed polarized microwave energy within the second harmonic frequency range of said primary waveguide, whereby coupling to and attenuation of the harmonic content of the microwave energy in said primary waveguide is enhanced.

2. The apparatus of claim 1 wherein said lossy termination means includes a plurality of resistive attenuative cards disposed diagonally across and within said secondary waveguides.

3. The apparatus of claim I wherein said primary waveguide is a section of rectangular waveguide having a pair of mutually opposed broad walls and a pair of mutually opposed narrow sidewalls, and wherein said array of secondary waveguides includes at least two rows of secondary waveguides disposed in side-by-side relation with said rows of secondary waveguides extending along the length of each said broad walls of said primary waveguide.

4. The apparatus of claim 1 wherein said secondary waveguides are of square cross section.

5. The apparatus of claim 4 wherein said primary waveguide is of rectangular cross section and the planes of the walls of said secondary square waveguides are disposed at substan tially 45 to the longitudinal axis of said primary waveguide.

6. The apparatus of claim 5 wherein there are three rows of square cross-sectional and two rows of triangular cross-sectional secondary waveguides coupled to each of said broad walls of said primary waveguide.

7. The apparatus of claim 6 wherein there are two rows of triangular cross section and one row of square cross section secondary waveguides coupled to each of the narrow side walls of said primary rectangular waveguide.

8. The apparatus of claim 6 wherein said lossy termination means includes a resistive card disposed in each of said secondary waveguides of triangular cross section with said resistive card being disposed with the plane of said card extending matching said primary waveguide to the source and to the load, respectively.

10'. The apparatus of claim 4 wherein the height of said primary waveguide is progressively decreased toward the center thereof from both ends thereof for impedance matching said primary waveguide to the source and to the load, respectively. 

1. In a microwave low-pass filter of the ''''leaky wall'''' type, means forming a hollow primary waveguide adapted to be connected between a source of high power microwave energy having substantial undesired harmonic content and a load for transmission of the primary waveguide energy within a certain passband of microwave energy to the load, means forming an array of secondary waveguides dimensioned to be below cutoff for the primary band-pass frequency energy and coupled through the walls of said primary waveguide to the microwave energy in said primary waveguide for extracting harmonic microwave energy of the primary guide therefrom, means forming a lossy termination for said secondary waveguides for absorbing the harmonic microwave energy coupled from said primary waveguide via said secondary waveguides, the improvement wherein, said secondary waveguides are rectangular hollow waveguides having at least nearly equal height and width dimensions such that said secondary waveguides will support crossed polarized microwave energy within the second harmonic frequency range of said primary waveguide, whereby coupling to and attenuation of the harmonic content of the microwave energy in said primary waveguide is enhanced.
 2. The apparatus of claim 1 wherein said lossy termination means includes a plurality of resistive attenuative cards disposed diagonally across and within said secondary waveguides.
 3. The apparatus of claim 1 wherein said primary waveguide is a section of rectangular waveguide having a pair of mutually opposed broad walls and a pair of mutually opposed narrow sidewalls, and wherein said array of secondary waveguides includes at least two rows of secondary waveguides disposed in side-by-side relation with said rows of secondary waveguides extending along the length of each said broad walls of said primary waveguide.
 4. The apparatus of claim 1 wherein said secondary waveguides are of square cross section.
 5. The apparatus of claim 4 wherein said primary waveguide is of rectangular cross section and the planes of the walls of said secondary square waveguides are disposed at substantially 45* to the longitudinal axis of said primary waveguide.
 6. The apparatus of claim 5 wherein there are three rows of square cross-sectional and two rows of triangular cross-sectional secondary waveguides coupled to each of said broad walls of said primary waveguide.
 7. The apparatus of claim 6 wherein there are two rows of triangular cross section and one row of square cross section secondary waveguides coupled to each of the narrow side walls of said primary rectangular waveguide.
 8. The apparatus of claim 6 wherein said lossy termination means includes a resistive card disposed in each of said secondary waveguides of triangular cross section with said resistive card being disposed with the plane of said card extending across the interior of said triangular secondary waveguide from the apex of the triangle to the base of said triangle.
 9. The apparatus of claim 4 wherein said array of secondary waveguides includes rectangular secondary waveguides at both ends of said primary waveguide which are of half the height of said secondary waveguides disposed intermediate the ends of said array of secondary waveguides for impedance matching said primary waveguide to the source and to the load, respectively.
 10. The apparatus of claim 4 wherein the height of said primary waveguide is progressively decreased toward the center thereof from both ends thereof for impedance matching said primary waveguide to the source and to tHe load, respectively. 